High-capacity computer storage devices typically include one or more electro-magnetic transducers and a corresponding number of magnetic media disks. The transducers, also known in the art as "heads," are adapted for transfer of electronic information between a data source, for example a computer, and data locations on the magnetic disks. Information is communicated in accordance with well-known conventions and formats that enable high-density storage, rapid access to data locations, high reliability, data integrity, and device miniaturization. A magneto-resistive (hereinafter referred to as MR) head is one of several types of electro-magnetic transducers known in the art. In general, an MR head includes an inductive component to write data and an MR sensor component to read data from magnetic media. In order to be active, the MR sensor requires an electrical bias current I.sub.B during the reading process. This bias current I.sub.B generally needs to be turned off during the writing process. FIG. 1 illustrates a simplified schematic of a prior art circuit 10 for generating a bias current I.sub.B for an MR sensor and for amplifying the underlying read signal produced by the MR head. FIG. 2 illustrates a number of curves representing various time varying voltages from the circuit 10 of FIG. 1.
In this prior art system, the MR sensor 12 is differentially AC coupled through capacitors 14 and 16 of value C to the read amplifier (READ AMP) 18. A biasing network including resistors 20 and 22 of value R supply DC biasing to the inputs of amplifier 18. Operational Amplifiers (OP AMPs) 24 and 26 provide the bias current through MR sensor 12 by developing a differential voltage across the series combination of resistors 28 and 30 and MR sensor 12 which has an equivalent resistance value of R.sub.MR. When the bias enable switch 32 is closed, the voltage developed by the DAC 34 is amplified by the OP AMPs 24 and 26 to produce voltage +V.sub.B at the top of R.sub.B 28 and -V.sub.B at the bottom of R.sub.B 30. This results in the bias current I.sub.B as follows: ##EQU1##
Because the voltage developed by the two Operational Amplifiers 24 and 26 is differential, the common-mode voltage V.sub.MR across the MR head 12 is close to ground potential so as to prevent electrostatic discharge (ESD) damage to the MR head. The absolute value of the bias current I.sub.B can be adjusted by changing the voltage at the output of DAC 34 to fit the precise MR sensor and magnetic media characteristics.
The Bias Enable Switch 32 turns the bias current I.sub.B off during the write operation and turns I.sub.B on during the read operations. Turning on I.sub.B at the beginning of the read operation produces an undesired voltage transient at the differential input terminals of the READ AMP 18, as illustrated by curve 56 in FIG. 2. The switch 32 closes at time t=T.sub.0, applying the output voltage of DAC 34 to the inputs of the OP AMPs 24 and 26, as shown by curve 50. The DAC output voltage is amplified to produce the differential voltage represented by curve 52 across the outputs of OP AMPs 24 and 26. This differential voltage (driven across the series network of two bias resistors 28 and 30 and the MR sensor 12) results in a bias current I.sub.B through the MR sensor 12. The current I.sub.B flowing through the MR sensor 12 develops voltage V.sub.MR represented by curve 54 across the MR sensor 12. A typical value of the MR sensor resistance (R.sub.MR) is 40 ohms, and a typical value of bias current I.sub.B is 10 mA; thus the voltage V.sub.MR developed across the MR sensor 12 when the switch 32 is closed may be expected to be on the order of 400 mV, with a relatively fast rise time because of a relatively small time constant. The inputs of the READ AMP 18 are capacitively coupled to the MR sensor 12 to block the DC voltage, while providing a path for a read signal from the MR sensor 12 having a bandwidth from a few hundred KHz (e.g., 300 KHz). To minimize distortion of low frequency components of the read signal, the time constant T.sub.C of the READ AMP input must be fairly large, on the order of 50 microseconds. The time constant T.sub.C may be determined from the following equation (approximately, considering that R.sub.MR is relatively small): ##EQU2##
After t=T.sub.0, a voltage transient is superimposed on the read signal from the MR sensor 12 across the inputs of the READ AMP 18, as represented by curve 56 shown in FIG. 2. The transient decays exponentially as expressed by the following equation: ##EQU3##
This decay is unacceptably long because it causes the head amplifier to saturate. Since a saturated head amplifier distorts the underlying read signal from the MR sensor 12, a significant portion of the magnetic track is wasted; the system can not effectively process the read signal until the transient sufficiently decays and the head amplifier returns to its linear operating region. It is therefore desirable to reduce the length of the transient to as short a duration as possible.
It is an object of the present invention to substantially overcome the above-identified disadvantages and drawbacks of the prior art.